Whooooo (sorry, it's a law of lazy feature writing) is responsible for owl depopulation? According to the BBC, some blame that meddlesome wizard kid:

Hope no one sees "Legend of the Guardians," then. But hold on: "people buying owls" is not necessarily a reason for owl-demise, is it? No one's taking them home and cooking them up for Hedwig stew. The article notes something else that might possibly be contributing to the de-owling more than Harry Potter fans:

That might have something to do with it, too.